1. Field of the Invention
The present invention relates to a method and an apparatus of producing optical fiber preform by use of a so-called outside vapor phase deposition method.
2. Background Art
Generally, in an apparatus of producing an optical fiber preform using a manufacturing method which is referred to as an outside vapor phase deposition method, both ends of a rod-shaped target element are held by a glass lathe or the like and the target element is rotated. Glass particles which are generated in the flame of the flaming burner used for generating glass particles are deposited on the periphery of the target element.
The target element may be removed in a subsequent step or may serves as a silica-based glass rod functioning as a core (region) in a case where an optical fiber is produced in a subsequent step.
A flame hydrolysis reaction or the like occurs in the flame due to introducing a glass source material gas into the flame of the burner together with a flaming gas and a supporting gas, and glass particles such as SiO2 are thereby generated.
The glass particles are deposited on the periphery of the rotated target element as described above.
By carrying out the aforementioned deposition step while performing traverse of the flaming burner used for generating glass particles in the axial direction of the target element, a deposited layer having deposited glass particles is formed on the periphery of the target element, and the deposition step is completed when the weight of the deposited layer reaches a predetermined weight.
The glass-particle-deposited body serving as a composite body constituted of the target element and the glass-particle-deposited layer formed in this manner is subjected to a heat treatment in a high-temperature furnace in a subsequent step, the glass-particle-deposited layer thereof is transparent-vitrified by sintering, and an optical fiber preform is obtained.
In the above-described apparatus of producing an optical fiber preform, deposition of glass particles may be carried out by sequentially performing traverse of a plurality of flaming burners used for generating glass particles in a single direction. In this case, after one burner (first burner) only performs glass particle generation and deposition in the period from the deposition start point to the deposition completion point, the burner deviates from the traverse step and moves so as to return the deposition start point so that the burner does not interfere with the other burner (second burner) performing deposition of glass particles and traverse.
In the returning period, it is necessary to make the flame of the burner as small as possible. For this reason, the flow rates of the flaming gas and the supporting gas are made as low as possible or the flow rate of the flaming gas is made as low as possible while closing the valve of the supporting gas in a conventional step.
However, in the case of making the flow rates of the flaming gas and the supporting gas as low as possible in the step of returning from the deposition completion point to the deposition start point as described above, inflammation occurs by the flame at the position close to the nozzle of the flaming burner, a nozzle end glows, and there is a problem in that the lifetime of the burner becomes extremely shortened.
With respect to the foregoing problems, for example, a method of stopping oxygen during a feedback step (for example, refer to Japanese Unexamined Patent Application, First Publication No. H04-170336), a method of supplying a purge gas into an oxygen nozzle during a feedback step (for example, refer to Japanese Unexamined Patent Application, First Publication No. H04-175239), or the like is proposed.
Additionally, as a method of inhibiting the end of an oxygen nozzle from being degraded, for example, a method of combining hydrogen with an inert gas or nitrogen (for example, refer to Japanese Unexamined Patent Application, First Publication No. H11-079774), a method of making the flow velocity high by reducing the thickness of the nozzle to be 1 mm or less (for example, refer to Japanese Unexamined Patent Application, First Publication No. H06-247722), a method of providing a seal layer around a nozzle used for supplying an oxygen gas (for example, refer to Japanese Unexamined Patent Application, First Publication No. S59-232933), or the like is proposed.
In particular, in the above-described Patent Documents, an oxygen gas corresponds to “supporting gas”, a hydrogen gas corresponds to “flaming gas”, and an inert gas or nitrogen gas corresponds to “purge gas”.
However, even where the methods are used which are disclosed in the aforementioned Japanese Unexamined Patent Application, First Publication No. H04-170336 and Japanese Unexamined Patent Application, First Publication No. H04-175239, the nozzle becomes temporarily high temperature at the moment of change in the flow rate of the gas, that is, at the moment of decrease in the flow velocity of the gas. The nozzle may be deformed due to reiteration of a high-temperature state and a low-temperature state.
Additionally, in the method disclosed in Japanese Unexamined Patent Application, First Publication No. H11-079774, deposition efficiency is affected depending on conditions of manufacture in no small part, there is a case where a preferable optical fiber preform cannot be produced.
In the method disclosed in Japanese Unexamined Patent Application, First Publication No. H06-247722, since the flow velocity decreases in the situation, for example, ignition, extinction, pilot burner, or the like, the nozzle glows.
Moreover, if the thickness of the nozzle is 1 mm or less, the deformation in the nozzle by action of the glowing nozzle becomes significant, and eventually, the lifetime thereof is shortened.
Furthermore, in the method disclosed in Japanese Unexamined Patent Application, First Publication No. S59-232933, deformation in the nozzle is avoidable; however, the size of the burner increases and the burner becomes complicated as a result of providing the seal layer, therefore, it is not desirable.
In addition, a degree of precision for manufacture the burner becomes deteriorated, the burner becoming excessively large, and deposition efficiency becomes reduced.